1. Field of Invention
This invention relates generally to a pressure sensing and control device, and, more particularly, to a device enabling the adjustment of purge gas flow automatically and dynamically during the orbital welding of tubular components, to maintain a desired internal tube pressure, for the purpose of affecting the internal geometry of a molten weld bead and reducing particle generation. Additionally, the present invention relates to a device enabling the adjustment of purge gas flow and pressure relative to the activity of an orbital tube welder, again for the purpose of affecting the internal geometry of a molten weld bead and reducing particle generation during the weld.
2. Description of Prior Art
When welding in abutment two metal tubes or components with tubular extensions, using an orbital welding device, such as the device disclosed in U.S. Pat. No. 5,196,664 to McGushion (1993), it is generally a desired result to create a weld bead which completely penetrates the walls of the tubular extensions at the seam formed by the abutment, has a weld bead which fully envelopes the inner diameter and outer diameter seam formed between the two tubular components, and fuses together the complete wall cross sectional areas of the tubular extensions.
Often, the result of complete penetration is an undesirable weld bead protrusion into the internal diameter of the tubular component, effectively decreasing this internal diameter at the weld location. This protrusion can trap impurities during normal operation of the manufactured product and create a significant and undesirable head loss in the system.
A method currently being used in industry to attempt the minimization of internal weld bead intrusion and, at the same time, fully penetrate the walls of the tubular components, involves the manual adjustment of the electric current supplied to the orbital welder prior to the welding process. Prior to the welding process, a welding practitioner attempts to adjust the electric current output of the orbital welding device to produce the desired weld bead, based on past experience. Depending on the configuration and heat transfer properties of the components being welded, significantly different schedules may be required. For example, the wall thickness of the tube is a significant factor in determining a weld schedule, and require quite dissimilar weld power schedules. A thin wall tube requires much less power to fully penetrate, compared to a thick walled tube.
The heat fusion process employed by orbital welders heats the outer diameter of the tube, melting the parent material. The heat travels through the wall of the tube on a convergent path to the inner diameter, with the resulting molten weld bead forming a similar path, converging to a point near the inner diameter. The resulting weld bead may not fully envelope the internal diameter weld seam, if there is even a slight misalignment of the seam versus the electrode of the orbital welder. This method can result in an unacceptable rate of weld rejection or failure.
Another method used in industry to attempt a solution to this problem involves adjusting the electric current being supplied to the orbital welding device and, simultaneously, increasing the internal purge gas pressure at the weld seam. The increased width of the weld bead, resulting from the increased electric current, insures complete penetration and full coverage of the seam between the tubular extensions, while the increase internal purge pressure supports the weld.
During the welding process, the highly viscous nature of the molten metal in combination with the increased bead width will cause the weld bead to sag down, due to the gravitational pull. To eliminate this undesirable sag, the internal volumes of the tubes are pressurized with gas to a pressure greater than the pressure of the gas surrounding the outer volume of the weld location. The positive internal pressure of the system tends to support the weld bead when in its molten phase, preventing the bead from sagging and minimizing internal weld bead intrusion.
The internal pressure increase is created by introducing a flow of inert gas into the inner volume of the components to be welded at an inlet upstream of the weld joint, and restricting the flow at an outlet downstream of the weld joint. The flow of gas can be kept constant in every stage of the welding process by employing a flow regulator, mass flow controller, fixed flow device, or a valve.
Although the concept of supporting the molten weld bead with increased internal pressure is approaching an acceptable solution, there are several major problems with the constant internal flow method. The first problem occurs if the internal pressure is too low, causing a sagging of the weld bead as previously described. The second problem occurs when the internal pressure is too high, causing the weld bead to bulge out radially, away from the center of the tubular extensions, characterized by an undesirable increase in internal and external diameters of the tubular extensions at the weld joint. If the internal pressure is even higher, the gas may cause the molten weld joint between the tubular extensions to rupture, completely destroying the weld bead and, possibly, the components being welded. A rupture is, for obvious reasons, an extremely undesirable occurrence. Any one of these problems may occur while using the constant flow positive internal pressure system.
The constant flow positive internal pressure system's inherent unpredictability is caused by the behavior of the tubes during the welding process. The orbital welding device creates an electric arc, flowing from the weld joint to an electrode attached to the welding device. The arc is generally first formed at a point which be considered, for the purpose of clarity, to be at an angle of 0 degrees. After an arc is formed, the electrode begins to rotate around the weld joint, melting the parent material of the tubes. Usually the electrode makes at least one 360 degree orbit.
As the electrode orbits around the tubular extensions, the parent metal of the component closest to the electrode melts, and quickly solidifies as the electrode rotates away. The fusion of the parent metal of the tubes, and the subsequent creation of a weld bead, causes that particular area of the seam to contract; and the remaining seam not yet fused, tends to separate, as an opposite reaction to the contraction of the welded area, forming a gap.
The temporary gap formed at this point allows the internal flow of gas to escape, causing an undesirable internal drop in pressure. When the electrode obits to the area of the seam gap, fusing this part of the seam, the internal pressure will again change, increasing to a pressure near or exceeding the pre-weld pressure. This phenomenon can degrade the quality of the weld or even rupture the weld seam, and potentially destroy the electrode.
Yet another method is used to try to compensate for the pressure drop due to the uneven tube contraction during a weld. The flow of purge gas inside the tubes is adjusted manually at the outlet restriction valve, located downstream of the components being welded, as disclosed in U.S. Pat. No. 5,824,983 to Huddleston (1998). The operator manually adjusts the valve throughout the welding process in order to maintain a near constant internal pressure.
In alternate variations on this concept, before the weld is initiated, the pressure of the system is calibrated by installing a calibration pressure gauge at the location of the seam to be welded, between the tubes. Additionally, a control pressure gauge is placed at a point downstream of the weld seam, upstream of the manual valve. A gas flow is then introduced into this calibration configuration. The flow is increased until the desired internal pressure, the pressure required to support the molten weld bead, is observed on the calibration gauge at the seam. Because the head loss between the weld seam location and the control gauge is generally constant throughout the welding process, the control gauge provides an accurate measurement of internal pressure, compensating for any head losses of the existing system, such as head losses due to changes in fluid path direction, diameter, or roughness. The pressure reading on the control gauge is observed and recorded as a reference for the welding process.
To prepare for the actual weld, the calibration gauge is removed, and the tubes are held together in abutment by a clamping device. The orbital welding device is then attached to the components and clamping device. During the welding process, it is required that a person continuously observes the pressure reading at the control gauge and adjust the metering valve in order to maintain the pressure recorded on the control gauge in the calibration mode.
Because the internal pressure of the system can change at a rapid rate, it is difficult for a person to accurately manually adjust the flow of internal gas. This inaccurate and slow method of adjustment can easily result in a flawed weld bead and damage to expensive components. In addition to the poor quality, low reaction time, and inaccuracies inherent to the manual pressure control system, it is labor intensive, requiring the constant supervision and control by a person.
The system for welding pipes disclosed in U.S. Pat. No. 5,304,776 to Buerkel (1994) provides positive internal pipe pressure and vacuum to counteract the gravitational affect on the molten weld bead. This requires a complicated system consisting generally of a mass flow controller (MFC), a vacuum generator, and a controller, working in conjunction with a pressure sensor. A closed loop control system is formed, with the MFC and vacuum generator responding to the pressure transmitted by the pressure sensor.
This system tends to react slowly to changes in the weld bead condition, due to the time required for a change implemented by the MFC and vacuum generator to complete fluid communication with the weld seam location. Weld bead conditions can change rapidly, and require a control loop system that can address developing problematic conditions before systemic failure.
Additionally, this system makes use of arc voltage control system loop between the controller and orbital welder. Essentially, the system may not be compatible with existing orbital welders presently in use, requiring the purchase of both the pressure control system and the compatible orbital welder.
What is needed is a device that facilitates the manufacture of predictable, high quality weld beads that do not protrude excessively into the internal diameter of the tubular extensions. What is also needed is a device that automatically, quickly, and accurately adjusts for pressure changes dynamically during the welding process.